1. Field of the Invention
The present invention relates to a mercury reduction system and a mercury reduction method of flue gas containing mercury that reduces mercury in flue gas discharged from a boiler or the like.
2. Description of the Related Art
In coal-fired flue gas and flue gas generated by burning heavy fuel oil may contain dust, sulfur oxide (SOx), and nitrogen oxide (NOx), as well as metallic mercury (Hg0). In recent years, various proposals have been made on methods and apparatuses for treating the metallic mercury, in combination with a denitration apparatus that reduces NOx and a wet desulfurization apparatus that uses an alkali absorbent as a SOx absorbent.
As a method for treating metallic mercury in flue gas, a system in which reduction denitration is carried out by spraying ammonium (NH3) into a flue in the upstream process of a high-temperature denitration apparatus, and oxidizing (chlorinating) mercury on a denitration catalyst to be aqueous hydrogen chloride, by spraying a chlorinating agent such as hydrochloric acid (HCl), and then reducing mercury by a wet desulfurization apparatus installed in the downstream side has been proposed (for example, see Patent Document 1).
FIG. 15 is a schematic of a flue gas treatment system including a mercury reduction system. As shown in FIG. 15, in a flue gas treatment system 100 including the mercury reduction system, a boiler 101 discharges flue gas 102 containing nitrogen oxide and mercury, and supplies to a reduction denitration apparatus 103, where nitrogen oxide is reduced. The heat of the flue gas 102 is exchanged with air by an air heater 104, and the flue gas 102 is supplied to a dust collector 106, after the heat is collected by a heat collector 105. A desulfurization apparatus 107 reduces sulfur oxide in the flue gas 102, and discharges as purified gas 108. The gas is then heated by a reheater 109 and discharged from a stack 110.
An NH3 injection spot 111 is provided upstream of the reduction denitration apparatus 103, and nitrogen oxide is reduced by NH3 supplied from an NH3 tank 112.
A hydrogen chloride concentration measuring unit 113 installed upstream of the desulfurization apparatus 107 in the flue measures the concentration of hydrogen chloride used as a mercury chlorinating agent, and a mercury concentration measuring unit 114 installed downstream of the desulfurization apparatus 107 measures the concentration of mercury. Based on the measured concentration values of hydrogen chloride and mercury, an operating unit 117 calculates the supply of an aqueous hydrogen chloride (HCl) solution 116 supplied from a hydrochloric acid solution tank 115. A controlling unit 118 controls the supply of evaporated hydrochloric acid (HCl gas) supplied into a flue 120 from an HCl injection spot 119 from the hydrochloric acid solution tank 115.
NH3, urea ((NH2)2CO), and the like are supplied as a reducing agent and HCl is supplied as a mercury chlorinating agent. Accordingly, on a denitration catalyst filled into the reduction denitration apparatus 103, NH3 promotes the reduction reaction of nitrogen oxide NOx in the flue gas 102 as the following formula (1), and HCl promotes the oxidation reaction of Hg as the following formula (2).4NO+4NH3+O2 4N2+6H2O  (1)Hg+½O2+2HCl HgCl2+H2O  (2)
In the conventional method, the reducing agent and the mercury chlorinating agent are not only supplied in a gaseous state as NH3 gas and HCl gas, but also supplied in a liquid state as an NH4Cl solution. When the agents are supplied in a liquid state as NH4Cl solution, NH4Cl is dissociated into NH3 gas and HCl gas. Accordingly, NH3 gas acts as a reducing agent and HCl gas acts as a mercury chlorinating agent.
Patent Document 1: Japanese Patent Application Laid-open No. 10-230137
However, if the reducing agent and the mercury chlorinating agent are supplied in a gaseous state as NH3 gas and HCl gas, there poses a problem that gas supply equipment such as a spraying nozzle is separately required for two systems.
When the reducing agent and the mercury chlorinating agent are supplied in a liquid state as NH4Cl solution, if NH4Cl is dissociated, NH3 gas and HCl gas generated from 1 mol of NH4Cl are 1 mol each. Accordingly, the generating amount of NH3 gas and HCl gas cannot be controlled. Consequently, if the required ratio of NH3 and HCl is not one-to-one, due to the gaseous nature of NOx and metallic mercury Hg0 in the flue gas, there poses a problem that at least one of NH3 or HCl may exceed or fall short.
When NH4Cl solution is sprayed into the flue, low-temperature droplets collide with structures such as a duct wall and a support member (beam) in the flue. Accordingly, the thermal strain is caused by thermal shock, and may damage the structures. Consequently, a spraying nozzle needs to be installed, so that the liquid droplets of the NH4Cl solution do not collide with the structures before evaporating. In this case, as shown in FIG. 16, for example, a duct is divided into a high concentration region A where the concentrations of NH4Cl solution, NH3 gas, and HCl gas are high, and a low concentration region B where the concentrations of NH4Cl solution, NH3 gas, and HCl gas are low, near the structure such as a duct wall. Consequently, there is a problem that the concentrations of NH4Cl solution, NH3 gas, and HCl gas near the structure such as the duct wall, become low.
The present invention is made in view of the foregoing, and has an object to provide a mercury reduction system and a mercury reduction method of flue gas containing mercury that can simultaneously and uniformly supply a reducing agent and a mercury chlorinating agent in any proportion with a simple device.